Production printing systems associated with data processing enterprises generally include a localized print controller within the printing system. The print controller controls the overall operation of the printing system including, for example, host interfacing, interpretation or rendering, and lower level process control or interface features of print engines of the printing system. However, prior to operation a printing system is typically calibrated.
Calibration entails having a given Reference curve, acquiring a Measurement curve depicting the printer's current state, deducing a Target curve from the Reference and Measurement curves by either normalizing the Measured and Reference curves to one another, or doing nothing to the Measured and Reference curves. Finally a Tone curve is created by comparing the Target and Measured curves in a way such that at each device value, x, the difference Target(x)−Measured(toneCurve(x)) is minimized.
Problems can arise in the deducing of the Target curve. For instance, if normalization of the curves is not performed the Tone curve may become clipped, which may either saturate too early or never saturate at all. The Tone curve may also lose considerable contrast in the low device value range (e.g. be too flat) in this case. If performing the standard normalization, local features of the Reference curve are preserved, but absolute measurements are not. Thus, the shape of features will be maintained, but their absolute reflectance will not be. This can lead to a change in the absolute contrast in the light or dark areas. If the peak optical density response of the Measured curve is less than the peak of the Reference curve (e.g., the printer cannot print as dark as ideally desired), the contrast across the entire printer output range will be degraded in order to accommodate the decrease in dynamic range.
Accordingly, a calibration mechanism is desired.